The histamine H4 receptor (H4R) is the most recently identified receptor for histamine (for reviews, see: Fung-Leung, W.-P. et al., Curr. Opin. Invest. Drugs 2004, 5(11), 1174-1183; de Esch, I. J. P. et al., Trends Pharmacol. Sci. 2005, 26(9), 462-469). The receptor is found in the bone marrow and spleen and is expressed on eosinophils, basophils, mast cells (Liu, C. et al., Mol. Pharmacol. 2001, 59(3), 420-426; Morse, K. L. et al., J. Pharmacol. Exp. Ther. 2001, 296(3), 1058-1066; Hofstra, C. L. et al., J. Pharmacol. Exp. Ther. 2003, 305(3), 1212-1221; Lippert, U. et al., J. Invest. Dermatol. 2004, 123(1), 116-123; Voehringer, D. et al., Immunity 2004, 20(3), 267-277), CD8+ T cells (Gantner, F. et al., J. Pharmacol. Exp. Ther. 2002, 303(1), 300-307), dendritic cells, and human synovial cells from rheumatoid arthritis patients (Ikawa, Y. et al., Biol. Pharm. Bull. 2005, 28(10), 2016-2018). However, expression in neutrophils and monocytes is less well defined (Ling, P. et al., Br. J. Pharmacol. 2004, 142(1), 161-171). Receptor expression is at least in part controlled by various inflammatory stimuli (Coge, F. et al., Biochem. Biophys. Res. Commun. 2001, 284(2), 301-309; Morse et al., 2001), thus supporting that H4 receptor activation influences inflammatory responses. Because of its preferential expression on immunocompetent cells, the H4 receptor is closely related with the regulatory functions of histamine during the immune response.
A biological activity of histamine in the context of immunology and autoimmune diseases is closely related with the allergic response and its deleterious effects, such as inflammation. Events that elicit the inflammatory response include physical stimulation (including trauma), chemical stimulation, infection, and invasion by a foreign body. The inflammatory response is characterized by pain, increased temperature, redness, swelling, reduced function, or a combination of these.
Mast cell degranulation (exocytosis) releases histamine and leads to an inflammatory response that may be initially characterized by a histamine-modulated wheal and flare reaction. A wide variety of immunological stimuli (e.g., allergens or antibodies) and non-immunological (e.g., chemical) stimuli may cause the activation, recruitment, and de-granulation of mast cells. Mast cell activation initiates allergic inflammatory responses, which in turn cause the recruitment of other effector cells that further contribute to the inflammatory response. It has been shown that histamine induces chemotaxis of mouse mast cells (Hofstra et al., 2003). Chemotaxis does not occur using mast cells derived from H4 receptor knockout mice. Furthermore, the response is blocked by an H4-specific antagonist, but not by H1, H2 or H3 receptor antagonists (Hofstra et al., 2003; Thurmond, R. L. et al., J. Pharmacol. Exp. Ther. 2004, 309(1), 404-413). The in vivo migration of mast cells to histamine has also been investigated and shown to be H4 receptor dependent (Thurmond et al., 2004). The migration of mast cells may play a role in allergic rhinitis and allergy where increases in mast cell number are found (Kirby, J. G. et al., Am. Rev. Respir. Dis. 1987, 136(2), 379-383; Crimi, E. et al., Am. Rev. Respir. Dis. 1991, 144(6), 1282-1286; Amin, K. et al., Am. J. Resp. Crit. Care Med. 2000, 162(6), 2295-2301; Gauvreau, G. M. et al., Am. J. Resp. Crit. Care Med. 2000, 161(5), 1473-1478; Kassel, O. et al., Clin. Exp. Allergy 2001, 31(9), 1432-1440). In addition, it is known that in response to allergens there is a redistribution of mast cells to the epithelial lining of the nasal mucosa (Fokkens, W. J. et al., Clin. Exp. Allergy 1992, 22(7), 701-710; Slater, A. et al., J. Laryngol. Otol. 1996, 110, 929-933). These results show that the chemotactic response of mast cells is mediated by histamine H4 receptors.
It has been shown that eosinophils can chemotax towards histamine (O'Reilly, M. et al., J. Recept. Signal Transduction 2002, 22(1-4), 431-448; Buckland, K. F. et al., Br. J. Pharmacol. 2003, 140(6), 1117-1127; Ling et al., 2004). Using H4 selective ligands, it has been shown that histamine-induced chemotaxis of eosinophils is mediated through the H4 receptor (Buckland et al., 2003; Ling et al., 2004). Cell surface expression of adhesion molecules CD11b/CD18 (LFA-1) and CD54 (ICAM-1) on eosinophils increases after histamine treatment (Ling et al., 2004). This increase is blocked by H4 receptor antagonists but not by H1, H2, or H3 receptor antagonists.
The H4R also plays a role in dendritic cells and T cells. In human monocyte-derived dendritic cells, H4R stimulation suppresses IL-12p70 production and drives histamine-mediated chemotaxis (Gutzmer, R. et al., J. Immunol. 2005, 174(9), 5224-5232). A role for the H4 receptor in CD8+ T cells has also been reported. Gantner et al., (2002) showed that both H4 and H2 receptors control histamine-induced IL-16 release from human CD8+ T cells. IL-16 is found in the bronchoalveolar fluid of allergen- or histamine-challenged asthmatics (Mashikian, V. M. et al., J. Allergy Clin. Immunol. 1998,101 (6, Part 1), 786-792; Krug, N. et al., Am. J. Resp. Crit. Care Med. 2000, 162(1), 105-111) and is considered important in CD4+ cell migration. The activity of the receptor in these cell types indicates an important role in adaptive immune responses such as those active in autoimmune diseases.
In vivo H4 receptor antagonists were able to block neutrophillia in zymosan-induced peritonitis or pleurisy models (Takeshita, K. et al., J. Pharmacol. Exp. Ther. 2003, 307(3), 1072-1078; Thurmond et al., 2004). In addition, H4 receptor antagonists have activity in a widely used and well-characterized model of colitis (Varga, C. et al., Eur. J. Pharmacol. 2005, 522(1-3), 130-138). These results support the conclusion that H4 receptor antagonists have the capacity to be anti-inflammatory in vivo.
Another physiological role of histamine is as a mediator of itch and H1 receptor antagonists are not completely effective in the clinic. Recently, the H4 receptor has also been implicated in histamine-induced scratching in mice (Bell, J. K. et al., Br. J. Pharmacol. 2004, 142(2), 374-380). The effects of histamine could be blocked by H4 antagonists. These results support the hypothesis that the H4 receptor is involved in histamine-induced itch and that H4 receptor antagonists will therefore have positive effects in treating pruritis.
Modulation of H4 receptors controls the release of inflammatory mediators and inhibits leukocyte recruitment, thus providing the ability to prevent and/or treat H4-mediated diseases and conditions, including the deleterious effects of allergic responses such as inflammation. Compounds according to the present invention have H4 receptor modulating properties. Compounds according to the present invention have leukocyte recruitment inhibiting properties. Compounds according to the present invention have anti-inflammatory properties.
Examples of textbooks on the subject of inflammation include: 1) Gallin, J. I.; Snyderman, R., Inflammation: Basic Principles and Clinical Correlates, 3rd ed.; Lippincott Williams & Wilkins: Philadelphia, 1999; 2) Stvrtinova, V. et al., Inflammation and Fever. Pathophysiology Principles of Diseases (Textbook for Medical Students); Academic Press: New York, 1995; 3) Cecil et al. Textbook Of Medicine, 18th ed.; W. B. Saunders Co., 1988; and 4) Stedman's Medical Dictionary.
Background and review material on inflammation and conditions related with inflammation can be found in articles such as the following: Nathan, C., Nature 2002, 420(6917), 846-852; Tracey, K. J., Nature 2002, 420(6917), 853-859; Coussens, L. M. et al., Nature 2002, 420(6917), 860-867; Libby, P., Nature 2002, 420, 868-874; Benoist, C. et al., Nature 2002, 420(6917), 875-878; Weiner, H. L. et al., Nature 2002, 420(6917), 879-884; Cohen, J., Nature 2002, 420(6917), 885-891; Steinberg, D., Nature Med. 2002, 8(11), 1211-1217.
Small-molecule histamine H4 receptor modulators according to this invention control the release of inflammatory mediators and inhibit leukocyte recruitment, and may be useful in treating inflammation of various etiologies, including the following conditions and diseases: inflammatory disorders, allergic disorders, dermatological disorders, autoimmune disease, lymphatic disorders, pruritis, and immunodeficiency disorders. Diseases, disorders and medical conditions that are mediated by histamine H4 receptor activity include those referred to herein.
2-Arylbenzimidazoles have been described as histamine H4 receptor modulators, See: U.S. Pat. Appl. Publ. 2005/0070550A1, U.S. Pat. Appl. Publ. 2005/0261309, U.S. Pat. Appl. Publ. No. 2007/0232616, and U.S. Pat. Appl. Publ. No. 2007/0244126. However, there still remains a need for potent histamine H4 receptor modulators with desirable pharmaceutical properties. Certain bicyclic heteroaryl-substituted imidazole compounds have now been found to have histamine H4 receptor-modulating activity.